FINALLY mention may be made of the wasps’ nest beetle par excellence, whose interesting life-history is briefly as follows. The eggs are laid by the parent beetle in the autumn in cracks of bark and crevices of rotten wood in places where it is likely wasps will come to collect their wood-pulp. The only point that is not quite cleared up is whether the eggs hatch in the autumn and the larvae hibernate in suitable hiding places, or whether they pass the winter in the egg state and hatch in the spring. The beetle has been induced to lay eggs in cap-
tivity, but the eggs never hatched; also it often happens with experiments that males and females are not caught or bred at the same time, and thus the fertility of the eggs is not assured.
The young larva; when first hatched are little active black mites with six legs. As soon as the wasp approaches the hiding place of the little larva? to obtain the paper for its nest, which it does by scraping the surface of the wood with its jaws, the beetle larva attaches itself to it and is carried by the unconscious wasp to its nest. It will thus be seen that there must be an enormous amount of mortality among the larva; in the cases where no wasps’ come to the tree, post, or fence where they lie hidden. But no doubt the female beetles lay a great number of eggs to counterbalance this, as do the oil beetles; the larva? of the oil beetles mount flowers and wait for a bee to come and carry them away to its nest.
On reaching the wasps’ nest our little larva at once bores into a wasp grub, and there lies bathed in its body fluids. The wasp grub is generally half-grown when the larva enters it. The larva should be looked for in the flank of the third or fourth abdominal segments, and as it is of a black colour it can be seen through the transparent skin of the wasp grub. It remains in the body of the grub until the wasp grub has closed the entrance to its cell with the usual silken cap. It now bores its way out of the wasp grub, changing its skin as it does so, and leaving the old skin as a plug to the hole through which it made its escape.
At this stage it is a fleshy, worm-shaped larva without any legs, and lies like a collar round the neck of the wasp grub. For a time it only sucks the juices of its victim, but eventually devours it altogether and changes to a pupa in the cell intended for the unfortunate wasp. When the silken cap of the cell is removed, the beetle is often found ready to emerge, and several beetles are often discovered occupying cells close to each other. When it has emerged from the cell, the beetle immediately leaves the nest, and probably dies as soon as the duties of paternity or maternity have been performed, since it is very rarely found outside the nest. It has, however, been taken on flowers and from under bark.
The writer has seen it leaving a nest, and also running across the root of a tree. Lacordaire recorded it at the sap of trees; but in this case it was probably a female beetle laying eggs, and as the mouth-parts are more or less rudimentary, it is doubtful if the beetle does much, if any, feeding in the perfect
state. There is a larger race of this beetle which feeds on the larva; of the queen wasps, but the majority only feed on the worker larvas.